Quite a few guides show how to disconnect the negative terminal of the battery but I needed to replace the battery in my 330i and I couldn't find any guides that explained how to remove it so here we are.

I understand that for many people this procedure will be self explanatory but I was a little apprehensive so there may be others in the same boat.

Tools that you'll need:
10mm socket
(possibly) 13mm socket
wrench for above sockets
flat head screwdriver

Translations
Boot = Trunk
Bonnet = Hood

The battery I used in this guide is the BMW 70mah. Diesel engined e9x's will no doubt have a higher rating and they may be larger. Check the top of the existing battery to see what rating it has before ordering one.

A couple of warnings before you go any further. First of all, you do all of this at your own risk. You're messing around with the electrical system of your expensive BMW, if you fry something then you foot the bill. In other words I take no responsibility for your actions. Secondly, the battery is quite heavy and contains some pretty nasty stuff so take care lifting it.

One more thing, open your car before carrying out any of this procedure. Don't just work with the boot open just in case you accidentally close the boot whilst the battery is disconnected. If that does happen and the car doors are open you can either bump the car from the terminals under the bonnet or climb through the seats into the boot - the choice is yours

You can get access to the battery by opening a panel on the right hand side of the boot. Turn the black plastic knob anti-clockwise and it should allow you to pull the entire section out.



Once opened you'll be presented with something like this:



What you're looking at is a protection bar that runs over the top of the battery to keep it in place if you roll the car. The large rectangular black plastic underneath the bar is is a safety device which can cut the connection to the positive terminal if an airbag is deployed. There's the negative terminal on the right hand side with a support to the right of the terminal and finally the big white thing is the battery

We need to remove the protection bar so use the 10mm socket and remove the two bolts that hold it in place. Remove the bar and bolts and put them somewhere safe.



There's another piece of plastic that holds the battery in place that you need to remove. I've highlighted the negative terminal (green) and the soft plastic (purple). Take the 10mm socket to the bolt that I've circled in purple and it should come undone fairly easy.



Take the bolt out and then remove the plastic piece and you'll have something like this.



Now you can remove the negative terminal. To do this you'll need that 10mm socket again to loosen the nut on the terminal. The thread on the bolt won't allow the nut to be removed completely (nice touch IMHO) so don't force it if it won't budge much after its been loosened.



Once you've loosened the terminal, remove it and place it where it can't touch the terminal again. Pushed down the side of the battery is fine where the soft plastic used to be. Just make sure that it can't pop back into place. Be mindful of this you really don't want it making its way back to the negative terminal.

Now the fun bit! The positive terminal is hidden away under that rectangle of black and red plastic. You can get to the terminal by flipping open the red plastic cover



This alone won't allow you to remove the terminal completely you also need to release that black plastic cover. To do that take a flat head screwdriver and place it into the lip and lift it away from the battery. This picture was taken after I'd removed it from the battery but the idea is the same.



Now the positive terminal is also disconnected you can remove the battery however it may be a bit tight with all those wires blocking the path.

To help you can separate the red terminal connector from the main black plastic cover. Lift up the flap and undo the nut with a 13mm socket.





This will split it into two halves. You can then remove the black wire that's linking the two halves by carefully pressing in the clip on the left hand side of the connector



Almost done! All you need to do now is remove the vent tube from the battery. Carefully pull it by the black plastic and it will come out.



You should now be able to remove the battery by lifting it by the two handles. Be careful, it's quite heavy.


Fitting the new battery

Fitting the new battery back in is just the reverse of the instructions, just make sure that you push the battery as far towards the front of the vehicle as possible.

One problem I had was putting the soft plastic back in place that fits next to the negative side of the battery. I found it easier to move the washer on the bottom of the bolt as high up the bolt as possible which keeps the bracket raised.



This is so I could see what I was doing and get the bolt into the hole. Once you've located it into the hole just tighten it enough to keep it upright and then squeeze the soft plastic between the bolt and the battery. Tighten up the bolt and check it's good and secure.

I won't go into detail for the remaining steps because the pictures above still apply. Here's what you need to do:

1. Fit the breather pipe, push it into the battery as far as it will go
2. Connect the positive terminal and tighten the bolt with a 10mm socket
3. If you split the terminal to allow easier access to the battery you can refit it now and don't forget to refit the black wire if you disconnected it.
4. Push down on the metal clips on the black plastic and they will click into place holding the cover down. Do this on both sides.
5. Refit the bar above the battery
6. Refit the negative terminal *

Start her up and set the clock. Job done. Well done

If you've any questions please reply to this thread instead of PM'ing me.

Thanks

Steve

* Some people say to switch on your lights before carrying out this step to prevent surges from the battery causing damage to components. I didn't do this and my car was fine. Besides I've got Xenons fitted and the surge may have damaged them instead so I never bothered